Add Cmux-style UI refinements with system colors

[PavloGrubyi]

Summary

• keep the Cmux-inspired sidebar, pane/tab, notification, spacing, and density refinements while restoring Limux's GTK/libadwaita system accent colors instead of hardcoded Cmux blue
• align UI font/icon settings defaults with the refined sidebar rows, pane tabs, notification panel, and pane/browser header controls
• add notification badge counts and regression coverage so limux_cmux_accent / rgb(0, 145, 255) cannot return unnoticed
• add the local dogfood workflow docs, Rust 1.92 toolchain pin, and local build status checker for verifying the active limux runtime

Testing

• cargo +1.92 fmt --check
• git diff --check HEAD~2..HEAD
• cargo +1.92 test -p limux-host-linux
• ./scripts/check.sh
• LIMUX_RUST_TOOLCHAIN=1.92 ./scripts/install-local-build.sh
• ./scripts/local-build-status.sh
• Xvfb launched the installed host with a seeded workspace and no GTK CSS parser errors; selected row sampled as srgb(78,51,42), not Cmux blue

Screenshot